Accuri Plant Ploidy

Using the Accuri C6 Flow Cytometer for Rapid and Accurate Analysis of the Nuclear DNA Contents of Flowering Plants

Summary. The nuclear DNA content (C-value) is a fundamental

most convenient and accurate way to determine C-values of plants, but sampling of the ca. 480,000 species of angiosperms

reported. For plant species for which C-values are available, these values span an extraordinary range, approximately 1250-fold (0.2 pg to 254.8 pg). For plants whose C-values are

the range of possible C-values exceeds the dynamic range of

for the unknowns and the appropriate controls within the

empirically, it considerably slows down the sample throughput.

Authors: David W. Galbraith and Georgina M. LambertUniversity of Arizona, Department of Plant Sciences and Bio5

White Paper

In determining C-values for new plant species, the Accuri C6

high dynamic range of the ADC (24 bits, providing 16 million

range of the described angiosperms. This white paper

cellular debris, and includes typical measurements from plant

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Flow cytometry is the simplest way to determine plant nuclear DNA contents (Galbraith et al., 1983). In order to allow the nuclei to pass through the flow cell, the plant tissues are homogenized by chopping, using a razor blade, in a suitable buffer. After filtration of the homogenate to remove large particulate materials, the samples are stained with DNA-specific fluorochromes and the fluorescence is quantified.

The absolute values for the amounts of the plant nuclear DNA are found by comparison to cells or nuclei of known DNA content, such as chicken red blood cells (Galbraith et al., 1983) or plant standards (Johnston et al., 1999; Dolezel et al., 2007; see also hyperlink c in Table 1). Standardization is complicated by base-pair specificity inherent to some fluorochromes (DAPI and Hoechst 33257 being AT-specific (Dolezel et al., 2007), and mithramycin and chromomycin A3 being GC-specific). The fluorescence produced by intercalating fluorochromes, of which propidium iodide (PI) is the most popular, are not biased by base-pair composition, and staining in the presence of RNAase eliminates interference by double-stranded RNA.

In this white paper, we provide detailed information as to how to use the Accuri C6 Flow Cytometer® for the analysis of plant genome sizes. The DNA histograms produced by the C6 display excellent peak CVs, and this instrument, uniquely, can provide simultaneous linear estimates of nuclear DNA content spanning at least two orders of magnitude (a range of 0.34-80.9 pg). This covers most of the C-values encountered in the flowering plants found across the world. A fuller discussion can be found at Galbraith (2009).

Materials. The seeds mentioned in this paper are available from Lehle Seeds (Round Rock TX 78680-2366 USA) and from J. Dolezel (Olomouc, Czech Republic).

Plant Production. We germinate all seeds, except those of Arabidopsis, in 3 inch plastic pots containing Scotts Metro-Mix 3000 (The Scotts Miracle-Gro Company, Marysville, Ohio) in a growth room at 25oC under a 12/12 light dark cycle with a light intensity of 150-175 μ.einsteins.m-2 sec-1; plants are watered daily. Samples are generally taken after four weeks of plant growth.

For Arabidopsis, we germinate seeds and grow plants under sterile conditions as described previously (Zhang et al., 2008). In brief, seeds are sterilized by immersion for 5 minutes in a freshly-prepared solution of commercial bleach diluted 1:1 with distilled water containing 10 μL/mL Triton-X100. The seeds are then rinsed 3x in sterile distilled water, and planted on Murashige and Skoog (MS) agar plates supplemented with 2% sucrose, and 1.2% agar. The seeds on the plates are vernalized by incubation at 4oC for 2 days before transfer to a Conviron growth chamber under a 12-h day/12-h night illumination regime, with an incident light flux of 150 to 175 μm m-2 sec-1 and a temperature of 22oC (day) and 20oC (night).

Preparation of Plant Homogenates and Labeling of Nuclei. All procedures are done at 4oC or on ice. Approximately 50-100 mg of plant tissue is excised and placed in a plastic 60 mm petri dish standing on a pre-chilled metal plate placed on ice in a rectangular plastic tray. We recommend the procedures all be done in a walk-in cold room. Galbraith’s buffer supplemented with 0.1% w/v Triton X-100 (Galbraith et al., 1983) is added in the proportion of 1.5 mL buffer per 100 mg tissue. Tissues are chopped using a new razor blade for 2-3 minutes. Each slicing action of the razor blade should cut cleanly and without bruising the tissue. The razor blade is discarded after a single use. The homogenate is filtered through the 30 µm nylon filter. The homogenate (0.5 mL) is added to a labeled tube containing 2.5 µL of 10 mg/mL DNAse-free RNAse A, and incubated on ice for 15 minutes. Propidium iodide is then added to a final concentration of 50 μg/mL. The stained samples are incubated on ice in darkness for 30 minutes prior to analysis.

Flow Cytometry. The Accuri C6 Flow Cytometer is switched on, and allowed to warm up for 15 minutes before analyses are begun. Waste and sheath fluid bottles are checked during this time, and are emptied or refilled, as required. The CFLow® software is started. Performance validation of the Accuri C6 Flow Cytometer is then done using 6- and 8-peak fluorescent bead mixtures (Spherotech) according to instructions (CFlow User Guide pp. 6-7). Finally, the Plant DNA Analysis Template is opened. This template consists of the following histograms: FSC-A vs SSC-A, FL1-A vs FL2-A, FL3-A vs FL2-A, and a univariate histogram of FL2A. The fluorescence signals (pulse area measurements) are screened by the following filter configurations: (a) FL-1: a 530/14 nm band-pass filter (b) FL-2: a 585/20 nm band-pass filter, and (c) FL-3: a 670 nm long-pass filter. Analysis of the homogenates is based on light-scatter and fluorescence signals produced from 20 mW laser illumination at 488 nm.

Introduction

Materials and Methods

The plates are kept in a vertical orientation so that the roots grow on the surface of, rather than within, the agar medium. Samples are harvested the day of the flow analyses.

We purchased plants of Astroemeria aurea (Lily of the Incas) from a local supermarket. The plants were supplied by Inter-American Products, Cincinnati, Ohio.

Propidium iodide is available from Calbiochem (EMD Chemicals, Gibbstown, NJ; Catalog number 537059), and a stock solution (1 mg/mL) in deionized water is prepared, and stored in aliquots at -20°C. RNAse A (10 mg/ml in water) is from Fermentas (Glen Burnie, MD; Catalog number EN0531), and is stored at 4oC. Nylon filters are from Partec (Swedesboro, NJ; Catalog number 04-0042-231); these have a notional porosity of 30 μm. Razor blades are available from VWR (VWR International, West Chester PA; Catalog number 55411-050).

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Species Common name 2C DNA content (pg) DNA content attribution

Arabidopsis thaliana ecotype Columbia

Thale cress 0.32 Lehle Seedsa

Raphanus sativus cv. Saxa Radish 1.11 J. Dolezelb

Medicago sativa L. cv. Cimarron

Alfalfa 3.50 Lehle Seedsa

Pisum sativum L. cv. Ctirad Garden pea 9.09 J. Dolezelb

Secale cereale L. cv. Dankovske Rye 16.19 J. Dolezelb

Triticum aestivum L. line 812

Wheat 34.65 Lehle Seedsa

Alstroemeria aurea Grah.

Lily of the Incas 80.90Inter-American

Productsc

Troubleshooting. Occasionally low yields of intact nuclei from the chopping procedure are encountered. This is usually due to the source tissue being of poor condition. Source tissues should be fresh, young, and well hydrated. Old or dried-out tissues do not generally provide histograms of adequate quality. Grasses are often problematic, and one may need to use more than the 200 mg sample size suggested in the protocol to provide sufficient nuclei for analysis. The tissues of some species can be refrigerated in damp paper towels for up to two days prior to analysis should this be necessary. Freezing tissues is not recommended.

Some plants may release mucilaginous compounds into the chopping buffer resulting in slime which should not be run through the instrument. Other species can release secondary plant products that reduce fluorochrome fluorescence. Careful observation of the samples during chopping is recommended; any changes in homogenate color and consistency are contraindications. If these are observed, alternatives are to employ different parts of the plant for analysis, and/or to include additives to the chopping buffer to absorb secondary products. Dolezel et.al. (2007) provide additional suggestions for sample preparation in these situations.

Poor CVs may be the result of excessive chopping, or chopping with dull razor blades. Many sources exist for satisfactory blades (VWR, Scientific, Fisher Scientific, Personna, America Safety Razor Company) which generally should have an edge thickness of 0.22 mm or finer. Double-edged shaving blades are

Plant homogenate samples are introduced to the SIP, and the RUN tab is clicked in CFlow to initiate data acquisition. Threshold levels are set empirically to eliminate from detection the large amounts of irrelevant debris that are found in plant homogenates. The default threshold values on the C6 are set at 80,000, and this setting will exclude from analysis very small nuclei. We set the threshold at 10,000 for FSC, with a secondary threshold of 1,000 for FL-2. These values are adjusted while acquiring data and observing the position of the nuclei on the bivariate dot plots such that all the nuclei are on scale with the least amount of debris appearing in these plots. The flow cytometer is routinely operated at the Slow Flow Rate setting (14 μL sample/minute), and data acquisition for a single sample should typically occupy 3-5 minutes.

The PI-stained nuclei will appear on the bivariate plots as a region of dots clustered around a diagonal line. This region will contain two populations of nuclei for non-endoreduplicated species; additional populations are seen if somatic endoreduplication occurs. A polygonal gate is drawn to enclose the nuclei, and univariate histograms of PI fluorescence (FL2-A) are accumulated based on this gate. The zoom tool is used to locate and isolate the peaks appearing within this univariate histogram. Regions of identification are then placed across the lowest peaks to export values representing peak positions and CVs. These data are used to calculate the DNA content of the nuclei.

Kew C-value Database (Bennett and Leitch, 2004)

Olomouc website (Dolezel et al., 2007)

Kew C-value Database (Bennett and Leitch, 2004)

Olomouc website (Dolezel et al., 2007)

Olomouc website (Dolezel et al., 2007)

Kew C-value Database (Bennett and Leitch, 2004)

Kew C-value Database (Bennett and Leitch, 2004)

Source of plants and hyperlinks

a http://www.arabidopsis.com/main/cat/seeds/teaching/Crop/!cps.htmlb http://lmcc.ieb.cz/research/protocols.php?protocols=dnac http://www.interamericanproducts.com

Table 1. Names of plant species, source of seeds, 2C DNA contents, and attribution.

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Results

The plants used in this study are identified in Table 1, along with their 2C DNA content values. A particularly useful resource is the C-value database maintained by Drs. Leitch and Bennett at The Royal Botanical Gardens, Kew (Bennett and Leitch, 2004).

Figure 1A shows the biparametric frequency distribution (FL2-A versus FL3-A) of a PI-stained homogenate prepared from pea (Pisum sativum) leaf tissue, as analyzed by the C6 Flow Cytometer. Pea has a reasonably large nuclear genome, and is known to produce high quality uniparametric DNA histograms (3). Two clusters are seen within this frequency distribution. One, roughly star-shaped cluster (arrow) which comprises most of the objects in the homogenate, corresponds to subcellular debris. No correlation is seen between the FL2 and FL3 signals within this cluster. The second cluster, boxed in red, forms a very minor proportion (1.8%) of the objects detected by the flow cytometer. Objects in this population exhibit a very strong correlation between the FL2 and FL3 signals, as would be predicted for PI-stained nuclei. If you place gates around this population (region P), the gated objects provide a very nice uniparametric DNA distribution with clearly-defined G0/G1 and G2 peaks. The peak positions in this case are 533,898.3 and 1,065,516.0 fluorescence units (a ratio of 1:1.996). The CVs for the G0/G1 peak are 2.1% (when determined using the CFlow software, after manually positioning windows at the 50% points on either side of the peak), and 2.6% (when determined using SigmaPlot, with fitting of a Gaussian function to the extracted data). This histogram is typical of the nuclear DNA contents found within non-endoreduplicated plant tissues (Galbraith et al., 1983, 1991).

ideal, except that one must find or devise suitable holders for these to avoid inadvertent self-mutilation. Unstable (drifting) peak positions may be due to insufficient periods of staining. In this white paper, we have recommended 15 minutes, but it is possible some species may require longer staining times. This can be easily monitored by flow analysis over time of successive samples taken from a single stained homogenate.

The Accuri C6 Cytometer and CFlow Plus have far exceeded our expectations. The C6 provides important additional data that we have previously been missing. One is the highly accurate volume determination, so that absolute cell counts are automatically recorded for each experiment. Secondly, the six-log scale means that we can collect all data without compensation, so that runs from different days can be more easily compared. Overall, we are extremely pleased with the C6, and would highly recommend it.

- Associate Professor of Immunology and Pediatrics

(A) Biparametric dot plot of FL2-A (585/40 nm) versus FL3-A (>670 nm) fluorescence emission.

(B) Uniparametric histogram of FL2-A fluorescence, gated on region P1 of panel A.

Figure 1. Flow analysis of homogenates from pea seedling tissue

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Figure 2A illustrates the corresponding biparametric frequency distribution obtained from leaf homogenates of Arabidopsis thaliana, which contains one of the smallest genomes of flowering plants (~157Mb (Bennett et al., 2003), or a 2C value of 0.32 pg). Arabidopsis exhibits endoreduplication within most of its somatic tissues and organs (Galbraith et al., 1991), and this produces multiple clusters within the frequency distributions, corresponding to an endoreduplicative series (2C, 4C, 8C, 16C, etc.). These clusters fall on the characteristic PI-DNA diagonal, which becomes particularly obvious following electronic expansion of the region in which it falls (Fig. 2B). Gating around this expanded region (P1, ~1.2% of the detected signals) provides uniparametric distributions with well-defined peaks (Fig. 2C) typical of Arabidopsis thaliana (Galbraith et al., 1991). The mean fluorescence values of the peak positions for the nuclei are 24541.9, 47446.1, 91698.5, and 179634.1, which fit a straight line almost perfectly (r2 = 0.9999); the CVs are also very acceptable (3.6%, 2.7%, 2.2%, and 2.4%). Similar results are seen for arabidopsis root homogenates (Figs. 2D, 2E). The nuclei comprise a slightly larger proportion of the detected signals (region P1), probably due to the absence of chloroplasts from roots. Gating on region P1 again provides uniparametric histograms of excellent quality and linearity (Fig. 2F): the fluorescence values of the peaks are 22773.6, 44930.8, 87703.1, and 171246.2 (r2 = 0.9999); the CVs were 3.5%, 2.8%, 3.1% and 3.4%.

Day-to-day reproducibility of the C-value measurements on the Accuri C6 is excellent. For seven repetitions on separate days, using different Arabidopsis seedlings as the source of nuclei, the mean FL2-A value of the 2C peak positions was 21,633.6 ± 1,588.5 (mean ± S.D.).

(A) Biparametric dot plot of FL2-A versus FL3-A fluorescence emission.

(B) Enlargement of the square region-of-interest (R1) containing the nuclei. The nuclei are indicated by polygonal region P1.

(C) Uniparametric histogram of FL2-A fluorescence, gated on region P1 of panel B.

Figure 2. Flow analysis of homogenates from arabidopsis leaf (A-C) and root (D-F) tissues

(D) As for panel A, except using roots.

(E) As for panel B, except using roots.

(F) As for panel C, except using roots.

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Figure 3. Simultaneous analysis of mixtures of plant homogenates

(A) Biparametric dot plot of FL2-A versus FL3-A fluorescence emission.

(B) Enlargement of the square region-of-interest (R2) containing the nuclei; the nuclei are enclosed by polygonal region P2.

(C) Uniparametric histogram of FL2-A fluorescence, gated on region P2 of panel B.

(D) Plot of DNA content versus the mean fluorescence values of the 2C peak positions for the four species.

(E) As for panel D except employing a log scale, with identification of the six DNA content peaks. Abbreviations: At, Arabidopsis thaliana; Ps, Pisum sativum; Ta, Triticum aestivum; Aa, Alstromeria aurea. 2C, 4C, 8C designate the C-values for the individual peaks within these species.

The next experiment extends the observed linearity of PI-based C-value measurements to a dynamic range that spans most of the values reported for flowering plants. Plant tissues are taken from four species (Arabidopsis thaliana, Pisum sativum, Triticum aestivum, and Alstroemeria aurea), chopped, filtered, and stained separately, then mixed. The composite biparametric frequency distribution contains the diagonal region characteristic of PI-stained nuclei (Figs. 3A, 3B). On magnification of the region, it is clear that the alstroemeria nuclei produce FL3-A signals that are off-scale, but these can be captured using a suitably-placed polygonal window (P2), and

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the resultant FL2-A uniparametric distributions have well-defined peaks corresponding to the nuclei of the different species (Fig. 3C). Assignment of the six peaks to nuclei of individual species is done from the results of accumulation of histograms produced for analyses of the unmixed homogenates of individual species. For Figure 3C, the individual peaks and their positions and CVs are: Arabidopsis 2C, 20868.2, 4.2%; Arabidopsis 4C, 40333.2, 3.5%; Arabidopsis 8C, 77941.5, 2.8%; Pisum 2C, 453340.4, 2.1%; Triticum 2C, 1584795.7, 2.8%; Alstroemeria 2C, 3486157.8, 2.4%. The peak position values are strongly correlated with the reported nuclear DNA content values (Figs. 3D, 3E; r2 >0.99).

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A further, general problem in C-value determinations relates to the choice of a standard against which to determine the 2C value for the unknown species. This conventionally has been an internal standard, ideally using a plant species having a C-value similar to that of the unknown, but not overlapping. Experience with other flow cytometers indicates data accumulation under conditions of linear signal amplification is preferred over logarithmic amplification, due to historical performance limitations of the latter. Given the extreme ranges of 2C values encountered in the plant kingdom, it can therefore be tricky to establish appropriate conditions and standards, for accurate flow analysis of nuclear DNA contents.

This problem is solved by the Accuri C6 in two ways. First, and remarkably, the linearity of measurement of plant genome sizes based on PI fluorescence extends over a dynamic range from 0.32 to 80.9 pg DNA (Figs. 3D, 3E). This is the first time this relationship has been demonstrated over such a large range. The upper limit to C-value measurements using the C6 will be about 370 pg DNA, based on the observed mean position for the 2C peak of alstroemeria (80.9 pg; channel 3486157.8) and the highest bin value available on the instrument (~16 x 106). This exceeds the largest record in the Kew C-value database by a factor of 1.5-fold. Extending polygonal gate P2 (Fig. 3) around the PI-DNA region downwards below the Arabidopsis 2C peak revealed little noise from debris for FL2-A values as low as 2,000. This value would correspond to a nuclear 2C DNA content of 0.032 pg, which is about six-fold smaller than the smallest record for the flowering plants (Bennett and Leitch, 2004; Zonneveld et al., 2005).

Second, the C6 displays a very high level of reproducibility in day-to-day measurements of the fluorescence of PI-stained plant nuclei. The variability in the position of the Arabidopsis 2C peak had a CV of 7.3%. This essentially obviates the need for internal standardization of DNA content values. Caution should be observed, nevertheless, since a number of trivial issues can alter the fluorescence emission detected by the Accuri, including issues relating to experimental manipulations (the staining time, and the presence of secondary products, for example), and those relating to the C6 itself (deposition of murky substances on the flow cell walls). Appropriate controls can readily handle these issues. Overall, this white paper indicates the C6 Flow Cytometer should be able to measure any plant 2C value without modification to the instrument, and without the requirement for even simple adjustments such as insertion of neutral density filters. The accuracy of measurement of the nuclear DNA contents, reflected by the low CVs, was consistently high across the entire range of measurements. We conclude that, for routine analysis of plant nuclear DNA contents, ploidy, or investigations of other issues requiring C-value determinations, the Accuri C6 Flow Cytometer provides an excellent measurement platform.

Discussion and General Troubleshooting

The nuclear DNA contents (C-values) of higher plants span a remarkable range of values (Bennett and Smith, 1976; Bennett and Leitch 2004), from 0.2pg (Fragaria viridis Duch.) to 254.8 pg (Fritillaria assyriaca Baker (Buitendijk et al., 1998)), or about three decades of DNA content. Nevertheless, C-value measurements are available only for about 2% of the species comprising the angiosperms. The Accuri C6 is optimally suited for the flow cytometric determination of plant nuclear DNA contents. This is for two main reasons: first, the C6 employs a 24-bit analog-to-digital converter (ADC) for signal processing, which provides an exceptional, six decade, dynamic range. This dynamic range is larger than that of the described 2C values for the flowering plants.

Second, the C6 is equipped with lasers that optimally excite PI for use as a DNA fluorochrome. This avoids base-pair bias inherent to DAPI, the Hoechst dyes, and mithramycin/chromomycin (Dolezel et al., 2007), but also provides a convenient way to deal with the large amounts of debris released from plant tissues by the chopping process (Galbraith et al., 1983). In comparison to situations typical in flow analysis of animal cell suspensions, in which the object of interest (the cell) represents most of the population, for analysis of C-values using plant homogenates, the objects of interest (the nuclei) are a very minor population. Plant debris both scatters light and can be autofluorescent. Since light scatter signals are, generally, used for triggering flow cytometric measurements, careful adjustment of thresholds is required to allow visualization of the nuclei; without appropriate thresholding, auto-rescaling obscures the presence of the nuclei on frequency distribution displays, and sample acquisition, based on total counts of scattering particles, arrests prior to accumulation of data for adequate numbers of nuclei. Autofluorescence produced by the photosynthetic organelles of aerial tissues can also overlap the emission of DNA-specific fluorescent signals and, since chloroplasts greatly outnumber nuclei within the cells of green tissue, this also can obscure the nuclei.

Using the C6, it is very simple to define the region of interest within biparametric distributions of FL2-A versus FL3-A since these detection channels roughly split the fluorescence emission of PI-DNA into shorter and longer wavelength spectral components (Figs. 1-3). Correlation between the fluorescence emissions within these two spectral bands produces an angled linear region within the biparametric contour plots containing discrete peaks of fluorescence, the lowest representing the 2C nuclei within G0/G1 cells (Figs. 1-3). Gating around this region provides very clean uniparametric histograms. Debris from different tissue sources, for example shoots (Figs. 2A, 2B) and roots (Figs. 2D, 2E) can be readily excluded for consideration, and mixing homogenates from different plant species (Figs. 3A, 3B) does not impede nuclear analysis.

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Trans Roy Soc London B 274B:227-274.

(~100 Mb) and Drosophilacytometry show genome size in Arabidopsis to be ~157 Mb and thus

MB. Ann Botany 91:547-557.

C-band polymorphism in Alstroemeria aurea, A. ligtu, and (Alstroemeriaceae). Plant Syst Evol 1998; 212:87-106.

Dolezel J, Greilhuber J, Suda J (2007).

Galbraith DW (2009). nuclear DNA contents over the full range of described angiosperm 2C values. Cytometry 75A:692-698.

Galbraith DW, Harkins KR, Knapp S (1991). Systemic endopolyploidy in Arabi-dopsis thaliana. Plant Physiol 96:985-989.

Galbraith DW, Harkins KR, Maddox JR, Ayres NM, Sharma DP, Firoozabady

Am J Bot 86:609-613.Zhang CQ, Barthelson RA, Lambert GM, Galbraith DW (2008).

more than 300 angiosperms. Ann Botany 96:229-244.

Literature Cited

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©2009 Accuri Cytometers, Inc.Accuri, C6 Flow Cytometer, CFlow, and Flow cytometry within reach are registered trademarks of Accuri Cytometers, Inc.

This white paper is abstracted in part from Galbraith (2009), with the permission of the author.

Notes

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